When ‘foo’ is used in connection with ‘bar’
it has generally traced to the WWII-era Army slang acronym
FUBAR (‘Fucked Up Beyond All Repair’ or
‘Fucked Up Beyond All Recognition’), later modified to
foobar. Early versions of the Jargon File
interpreted this change as a post-war bowdlerization, but it it now seems
more likely that FUBAR was itself a derivative of ‘foo’ perhaps
influenced by German furchtbar (terrible)
— ‘foobar’ may actually have been the
original form.

For, it seems, the word ‘foo’ itself had an immediate
prewar history in comic strips and cartoons. The earliest documented uses
were in the Smokey Stover comic strip published from
about 1930 to about 1952. Bill Holman, the author of the strip, filled it
with odd jokes and personal contrivances, including other nonsense phrases
such as “Notary Sojac” and “1506 nix nix”. The
word “foo” frequently appeared on license plates of cars, in
nonsense sayings in the background of some frames (such as “He who
foos last foos best” or “Many smoke but foo men chew”),
and Holman had Smokey say “Where there's foo, there's
fire”.

According to the
Warner Brothers Cartoon Companion Holman claimed to have found the
word “foo” on the bottom of a Chinese figurine. This is
plausible; Chinese statuettes often have apotropaic inscriptions, and this
one was almost certainly the Mandarin Chinese word
fu (sometimes transliterated
foo), which can mean
“happiness” or “prosperity” when spoken with the
rising tone (the lion-dog guardians flanking the steps of many Chinese
restaurants are properly called “fu dogs”). English speakers'
reception of Holman's ‘foo’ nonsense word was undoubtedly
influenced by Yiddish ‘feh’ and English ‘fooey’ and
‘fool’.

Holman's strip featured a firetruck called the Foomobile that rode on
two wheels. The comic strip was tremendously popular in the late 1930s,
and legend has it that a manufacturer in Indiana even produced an operable
version of Holman's Foomobile. According to the Encyclopedia of American
Comics, ‘Foo’ fever swept the U.S., finding its way into
popular songs and generating over 500 ‘Foo Clubs.’ The fad left
‘foo’ references embedded in popular culture (including a
couple of appearances in Warner Brothers cartoons of 1938-39; notably in
Robert Clampett's “Daffy Doc” of 1938, in which a very early
version of Daffy Duck holds up a sign saying “SILENCE IS
FOO!”) When the fad faded, the origin of “foo” was
forgotten.

One place “foo” is known to have remained live is in the
U.S. military during the WWII years. In 1944-45, the term ‘foo
fighters’ was in use by radar operators for the kind of mysterious or
spurious trace that would later be called a UFO (the older term resurfaced
in popular American usage in 1995 via the name of one of the better
grunge-rock bands). Because informants connected the term directly to the
Smokey Stover strip, the folk etymology that connects it to French
“feu” (fire) can be gently dismissed.

The U.S. and British militaries frequently swapped slang terms during
the war (see kluge and kludge
for another important example) Period sources reported that
‘FOO’ became a semi-legendary subject of WWII British-army
graffiti more or less equivalent to the American Kilroy. Where British
troops went, the graffito “FOO was here” or something similar
showed up. Several slang dictionaries aver that FOO probably came from
Forward Observation Officer, but this (like the contemporaneous
“FUBAR”) was probably a backronym .
Forty years later, Paul Dickson's excellent book “Words”
(Dell, 1982, ISBN 0-440-52260-7) traced “Foo” to an
unspecified British naval magazine in 1946, quoting as follows:
“Mr. Foo is a mysterious Second World War product, gifted with bitter
omniscience and sarcasm.”

Earlier versions of this entry suggested the possibility that hacker
usage actually sprang from FOO, Lampoons and Parody,
the title of a comic book first issued in September 1958, a joint project
of Charles and Robert Crumb. Though Robert Crumb (then in his mid-teens)
later became one of the most important and influential artists in
underground comics, this venture was hardly a success; indeed, the brothers
later burned most of the existing copies in disgust. The title FOO was
featured in large letters on the front cover. However, very few copies of
this comic actually circulated, and students of Crumb's
oeuvre have established that this title was
a reference to the earlier Smokey Stover comics. The Crumbs may also have
been influenced by a short-lived Canadian parody magazine named
‘Foo’ published in 1951-52.

An old-time member reports that in the 1959 Dictionary of
the TMRC Language, compiled at TMRC,
there was an entry that went something like this:

FOO: The first syllable of the sacred chant phrase “FOO MANE PADME
HUM.” Our first obligation is to keep the foo counters turning.

(For more about the legendary foo counters, see
TMRC.) This definition used Bill Holman's nonsense
word, then only two decades old and demonstrably still live in popular
culture and slang, to a ha ha only serious analogy
with esoteric Tibetan Buddhism. Today's hackers would find it difficult to
resist elaborating a joke like that, and it is not likely 1959's were any
less susceptible. Almost the entire staff of what later became the MIT AI
Lab was involved with TMRC, and the word spread from there.